Epoxy resins constitute a broad class of polymeric materials having a wide range of physical characteristics. The resins are characterized by epoxide groups which are cured by reaction with certain catalysts, or curing agents, to provide cured epoxy resin compositions with certain desirable properties. The most commonly used curing agents are aliphatic amines such as diethylenetriamine, triethylenetetramine and the like.
When cured epoxy resins with improved heat and chemical resistance properties are desired an aromatic diamine such as 4,4'-methylene dianiline and m-phenylenediamine have been used. Prior to the invention described in U.S. Pat. No. 3,427,282 aromatic diamines and polyaromatic diamines had limited utility as epoxy curing agents because they were invariably solid materials which were mixed with the vicinal epoxides with considerable difficulty. The above-mentioned patent disclosed a curing agent for epoxy resins that could be used to eliminate some of the prior handling difficulties encountered with aromatic diamines. This curing agent consisted of 4,4'-methylene-bis(2-alkylanilines) wherein the alkyl substituent was an ethyl group or a secondary alkyl group having three to six carbons. It was found that curing agents composed of this material were readily blendable with liquid resins because the curing agents were either liquid, or solids which readily dissolved in the resins at processing temperatures. The use of this material as an epoxy resin curing agent gave cured resins having a lower heat distortion temperature than those cured with 4,4'-methylene dianiline and m-phenylenediamine.
U.S. Pat. No. 3,917,702 describes a stepwise process for preparing a polyaromatic amine epoxy curing agent that was readily blendable with epoxy resins and produced a cured resin having a higher heat distortion temperature than those cured with previously known polyaromatic amines. This process involved: (1) dissolving an aromatic polyamine in a solvent, (2) adding an active aldehyde compound to the solution in a mole ratio of polyamine to aldehyde in the range of from 0.75 to 5 and maintaining the temperature in the range from 25 to 100.degree. C. until the reaction is complete, (3) removing the water from the reaction mixture, (4) adding an aromatic diamine in equal molar proportions to that used in the initial reaction in Steps 1 and 2, and (5) removing water formed in Step 4 by heating at reduced pressures to 150.degree. C.
U.S. Pat. No. 3,459,781 discloses a method for producing liquid polyisocyanates that involves the production of a polyaromatic amine precursor. The polyaromatic amine precursor is produced in a step-wise reaction that consists of (1) reacting formaldehyde with an aromatic monoamine in the presence of a mineral acid catalyst, (2) after the reaction of Step 1 is complete, an aromatic diamine is added to the reaction mixture of Step 1 in such an amount as to be 3.3 to 80 mole percent of the aromatic monoamine of Step 1 and the reaction is continued below 50.degree. C., (3) and heating the reaction product of Step 2 to at least 60.degree. C. for a period of time sufficient to complete the reaction. Usually attempts to increase the heat distortion, or deflection, temperatures were accompanied by an increase in the working viscosity of the resin while curing. This adversely affects the utility of the material for many applications.
Similar methods for preparing a polyaromatic amine precursor for the preparation of liquid isocyanates may be found in U.S. Pat. Nos. 3,012,008; 3,492,251; 3,936,483; and 4,071,558.